1. Field of the Invention
The present invention generally relates to electronic circuit modules which are intended for mounting in a slot of a mechanical structure which provides electrical interconnection thereto and, more particularly, to electronic circuit board modules containing printed wiring boards.
2. Description of the Prior Art
So-called printed circuit boards (PCB) or printed wiring boards (PWB) have been known for many years and may be formed by many techniques (e.g. screening, plating, etching, etc.). Printed wiring boards provide for a compact, structurally robust and easily manufactured electronic circuit structure. Complex interconnection patterns may be formed using multiple metal layers interleaved with insulating layers in the board structure. Discrete electronic components and integrated circuits (or sockets intended to receive connections thereto) are affixed to the boards by soldering leads thereof to metallized pads on a surface of the board or inserting leads through holes in the printed wiring board.
As electronic systems, such as communication systems and data processors, have become more complex, however, the use of multiple printed circuit boards connected to each other has been convenient for accommodating such systems in a compact enclosure or other physical arrangement and limiting the size of printed wiring boards to limit the potential for damage from vibration, impact and the like. Additionally, for ease of maintenance and repair, it has become common to fabricate complex systems in a modular form, often of standardized dimensions and to mount the modules in an interconnection structure which also mechanically supports the printed wiring boards. Such structures may be of various forms referred to as card cages, backplanes, sub-racks and the like. As these arrangements for providing interconnection and mechanical support for the modules have become more fully developed, they have also become largely standardized, placing limits on the size and shape which can be occupied by a module, including a printed wiring board and components mounted on the printed wiring board.
At the same time, functionality and integration density of integrated circuits has increased. Accordingly, the functionality and parts count of such modules has also increased both to exploit commercially available hardware dimensions and to avoid increasing complexity and relative weight of the support and interconnection structure, as would occur if modules were made smaller. Additionally, maintaining constant module size allows systems to be updated as improvements are made in individual modules. Moreover, as circuit cycle and response speeds have increased with increased integration density, increased module functionality allows connection length and signal propagation time within a module to be minimized.
Particularly in this last regard, it has become desirable to further increase module performance while increasing module functionality by further increasing parts count in the modules to the extent possible within the allowed volume of the module. Unfortunately, the limiting factor is generally the area of the printed wiring board that may be accommodated in a module of the mechanical support and interconnection arrangement employed. Efforts to include additional printed wiring boards in a module have usually involved mechanical and/or electrical connection of two boards but which are received in the support structure in the same manner as if the boards were separated and in different modules. U.S. Pat. No. 4,107,760 to Zimmer and U.S. Pat. No. 5,396,401 to Nemoz are typical of such arrangements.
Further, heat dissipation requirements have increased with increased switching speed and functionality as discussed in U.S. Pat. No. 5,483,420 to Schiavini which provides a heat sink plate in a single board module. In this regard, plural boards in a module have often required complex heat sink structures such as that disclosed in U.S. Pat. No. 4,916,575 to Van Asten.
At the present time, a prevalent standard for modular circuit packaging is commonly known as the "Versa Module Europa" (VME) which incorporates a number of design standards including those known in the art as IEEE standard 1101.1, IEEE standard 1101.2, VITA20, ANSI/VITA1, VITA1.1 (VME64X) and IEEE standard P1386.
While these standards allow some degree of freedom in the mechanical design and component layout within the module, all of the standards involved in the collective VME standard are directed to modules including only a single printed wiring board having components mounted on only a single side thereof. The placement of the board in modules in accordance with the VME standard allows a component height of approximately 0.520 inches on one side of the single board while allowing a very low component height/pin protrusion height of approximately only 0.075 inches on the opposite "connection" side of the board. This very limited pin protrusion height precludes mounting of most common integrated circuit components on the connection side of the board. (Modern surface mount technology (SMT) components generally have a height between 0.090 and 0.160 inches.) Therefore, the area available for component mounting is currently a major impediment to increasing module functionality and parts count. By the same token, since the VME standard contemplates only a single board per module and module mounting at 0.800 inch pitch, the volume available for employment of heat removal structures is also quite limited.
It is also known, as discussed in U.S. Pat. No. 4,879,634, to augment the area of the single VME standard board with an additional board known as a mezzanine board. (When a mezzanine board is present, the main board of the VME module is referred to as the "host" board since the mezzanine board essentially plugs into the host board. The term "host" is also used synonymously with "main" board whether a mezzanine board is employed in a module or not.) Input and output (I/O) connections to the mezzanine board from the interconnection structure are made through the host board. This configuration increases the necessary length of connection paths and also places an extra plug and socket or other unsoldered connection arrangement in the signal path which may be a source of noise or signal path discontinuities, possibly intermittent with vibration, acceleration or differential thermal expansion.
Further, while the mezzanine board has been used successfully in many module designs, it can be readily seen that it occupies volume that could otherwise be used for heat removal structures such as heat sinks. Moreover, the mezzanine board is commonly attached to the host board, in accordance with the VME collective standard, such that the component side of the mezzanine board generally faces the component side of the host board and thus tends to aggravate the occurrence of hot spots on the host board. The components on the mezzanine board may interfere with air circulation over the surface of any heat sink which may be provided while the proximity of components on the mezzanine board to the host board provide localized heat input thereto. Modification of such a heat sink may also be required to accommodate components on the mezzanine board and in any case, the area and coverage of the heat sink must be reduced to accommodate connections to the host board.
In summary, a mezzanine board can be used to supplement the area of a single printed wiring board host provided in accordance with the VME standard but is not an ideal solution since its use increases length of signal and power connections, requires connections which are not soldered and interferes with heat sinking of the host board while increasing the likelihood that hot spots will develop on the host board. Further, due to the unsoldered connections, the difficulty of mechanically supporting a mezzanine board, the requirement that connections to the mezzanine board be made through the host board connectors and the volume which is occupied by a mezzanine board, only one mezzanine board can be used with a host board; limiting the total module area to less than twice the host board area. However, the use of a mezzanine board has been the only practical structure to approach the problem of supplementing printed wiring board area within the rules of the VME standard.
The principal reason that the mezzanine board has been the only practical approach to increasing module functionality and parts count is the geometrical configuration of host board connectors in accordance with the VME standard. Specifically, to obtain a sufficient number of contact pins, an entire edge of the VME printed wiring board is populated with connection pins grouped into three connectors. As the VME standard was developed, commercially available edge connectors were employed. Two of the connectors which are adjacent corners of the VME printed wiring board (designated the P1 and P2 connectors) included three or five rows of pins while the central connector (designated P0) included five rows of I/O pins with an additional two rows of ground pins.
As commercial connectors having these configurations were mounted on the component side of the VME printed wiring board, the center line of the central P0 connector was thus offset from the center line of the P1 and P2 connectors (by 1.85 mm or 0.073 inches). This offset also, in time, became part of the VME standard and could not be met by connectors attached to more than one printed wiring board of a module. Therefore, the VME standard could not be met for a second printed wiring board unless connections were provided through a host board, as in the mezzanine board configuration discussed above. No further augmentation of printed wiring board area with additional mezzanine boards is possible for the reasons discussed above and therefore even the non-ideal mezzanine board solution is limited to less than doubling the available area of the host board while presenting further problems which may compromise performance of the module or reliability of a module in severe environmental conditions of vibration, acceleration or thermal excursions.